1. Field of the Invention
Embodiments of the present invention relate to the field of circuits for protecting an inductance so that the current flowing through it does not exceed its saturation current, and relate to any system in which the current is desired to be limited in an inductance. An example application for embodiments of the present invention is the field of power converters (step-up or step-down) in which a switch controlling the current in an inductance is controlled by a train of pulses, for example, modulated in width (PWM), in frequency (FWM), etc.
2. Discussion of the Related Art
FIG. 1 very schematically shows a conventional example of a step-up converter.
Such a converter uses an inductance L in series with a diode D between a terminal 1 for applying a D.C. input voltage Vin and a terminal 2 for applying an output voltage Vout for a load 3 (Q). A capacitor Cout for storing voltage Vout is connected between terminal 2 and ground M. Further, a switch K (for example, a MOS transistor) connects the anode of diode D (unction point of the inductance and of the diode) to ground. Switch K is controlled by a width-modulated pulse train to control the output voltage based upon the needs of the load or a predetermined value. The operation of such a converter is known. When switch K is on, a current flows through inductance L from voltage source Vin (for example, a battery) while capacitor Cout supplies load 3. When switch K turns off, the power stored in inductance L recharges capacitor Cout at the same time as it supplies the load.
To protect inductance L against a deterioration, it must be ascertained that the current flowing therethrough does not exceed its saturation current from which the inductance behaves as a wire. For this purpose, the turning-on of switch K is conventionally conditioned by a current threshold in the inductance or in the switch.
FIG. 2 shows a conventional example of an integrated circuit 10 for controlling a PWM converter integrating an inductance protection function. It shows, integrated to this circuit, switch K (in dotted lines) between two terminals 11 and 12 of circuit 10 respectively connected to the anode of diode D (here, a Zener diode) and to ground M. To control output voltage Vout, circuit 10 also comprises a terminal 13 of connection to terminal 2 of the converter. Circuit 10 is supplied from D.C. voltage Vin filtered by a capacitor Cin, terminal 1 being connected to a terminal 14 of circuit 10.
In the example of FIG. 2, the converter is intended to supply light-emitting diodes 20 (LEDs) ensuring a screen backlighting function (for example, of a mobile phone). Circuit 10 then integrates, optionally, a second switch K for turning on/off the load formed of diodes 20 in series. Switch K′ shown in dotted lines in the form of a MOS transistor connects a terminal 15 of circuit 10 connected to load 20 to a terminal 16 of this same circuit connected to ground M, generally via a protection resistor Rp. Of course, switch K′ may be external to circuit 10.
Circuit 10 comprises means not shown for measuring the current in inductance L. This current measurement is generally induced from the voltage across the inductance, measured at terminals 11 and 14 of the circuit. Another method consists of measuring (for example, by means of a shunt) the current in switch K. Circuit 10 compares the information linked to the current in inductance L with a reference value to force the turning-off of switch K in case said value is exceeded, independently from the PWM digital signal control point provided on a terminal 17.
A first family of known circuits comprises a predetermined reference, internal to circuit 10. A disadvantage of such a solution is that circuit 10 is then dedicated to an inductance L or, to stand several inductances of different values, requires setting of a minimum threshold while some inductances could stand higher currents.
A second solution consists of providing a resistor Rs connected to an additional terminal 18 of circuit 10 and to ground M to parameterize the limiting threshold of circuit 10. Such a solution enables changing resistance Rs when the value of the limiting current is desired to be modified, for example, after an inductance change. A disadvantage of this solution is that it requires a terminal (18) of additional connection of integrated circuit 10 as well as an external resistor.
FIG. 3 illustrates the variation of the maximum tolerable or standable current (ILmax) according to value L of an inductance. The curve, the exact shape of which depends on the considered inductance family (materials, conductor diameter, power dissipation capacity, etc.) shows a decrease in the maximum standable current along with the increase in the inductance value. Accordingly, when a limiting threshold is desired to be set in a circuit 10, be it internally or externally, so that it can stand several different inductances L, a relatively low threshold Imax has to be set to be able to protect all the inductances in the range. As a result, according to the inductance connected to circuit 10, its capacities are not fully exploited.
FIGS. 4A and 4B illustrate the operation of a circuit such as shown in FIG. 2 for two different inductance values L1 and L2. The left-hand portion of the timing diagrams of FIGS. 4A and 4B has been drawn for an inductance of relatively high value L1 with respect to the value of a relatively low inductance L2, the operation of which is illustrated in the right-hand portion of the timing diagrams. FIG. 4A illustrates current IL in the inductance while FIG. 4B illustrates the OFF and ON periods of switch K It should however be noted that this applies to the case of a step-up converter such as illustrated in FIG. 2, the off and on periods of the switch being inverted in the case of a step-down converter.
To enable operation with inductances L1 and L2, limiting current Imax set by circuit 10 is a function of the inductance of lower value L1. As illustrated in the right-hand portions of the timing diagrams, this results in inductance L2 having its limiting current ILmax greater than current Imax is not fully exploited. Indeed, even admitting that the duty cycle of the turn-on pulses of switch K is set in the same way as for low-value inductance L1, the limiting function is activated as soon as current I exceeds value Imax, while the control of the converter with an output voltage control point Vout would have required a longer power storage period.
The need to have an integrated circuit for controlling a converter that can accept several different inductance values is more and more frequent. Indeed, the manufacturers of integrated control circuits 10 are generally distinct from inductance manufacturers which will assemble the inductance and circuit 10 in a converter.